1. Field of the Invention
The present invention is directed to a method for determining the curve for a gradient current in a magnetic resonance tomography apparatus, as well as to a magnetic resonance tomography apparatus operating according to the method.
2. Description of the Prior Art
In MR tomography devices, pulse sequences are employed which are generated by gradient currents. Calculating the curve of the gradient current is very complicated. This is particularly true when the k-space is not scanned in straight sections, but instead a general k-space scanning strategy is used. An example of such a sequence is the spiral-imaging method disclosed in the U.S. Pat. No. 4,651,096, wherein the k-space path exhibits a spiral shape.
In order to attain maximum performance of the MR tomography device, effort is made to optimally utilize the gradient system with respect to the slew rate and gradient amplitude. The calculation of complicated differential equations is required for the determination of the curve of the gradient current with these prescriptions as boundary conditions. It would greatly exceed the computing capacity of the currently customary control computers to calculate these equations in realtime during the measurement. For MR devices as are offered, e.g. by Siemens under the name "Magnetom".RTM., it is therefore customary to calculate a table offline with support values for the curve of the gradient current and to store as the reference current curve for a class of pulse sequences (for example, for the aforementioned spiral-imaging method). Given a change of the measurement field, only the gradient amplitude is then scaled. Since it must be assured that the allowable slew rate is not exceeded, however, one cannot attain an optimal utilization of the gradient system. The reference current curve must be determined such that the scaling does not cause the limit values of the slew rate for any adjustable measurement field to be exceeded.